Preparation of photographic silver halide emulsions utilizing polyoxalkylenated gelating derivatives

ABSTRACT

1,130,610. Preparing silver halide emulsions. GENERAL ANILINE &amp; FILM CORP. Nov. 23, 1965 [Nov. 27, 1964], No. 49749/65. Heading G2C. Silver halide emulsions are obtained by mixing an aqueous solution of a water-soluble silver salt and an aqueous solution of one or more water-soluble halides in the presence of an aqueous solution of a water-soluble polyoxyalkylenated derivative of gelatine, flocculating the resulting mixture and separating the flocculate. The flocculate may be washed and redispersed in gelatine. The polyoxyalkylented derivatives of gelatine are obtained by reacting gelatine with an alkylene or substituted alkylene oxide e.g. ethylene oxide, propylene oxide, butylene oxide, styrene oxide and cyclohexene oxide. The emulsion may be digestedbefore flocculation and flocculated by simply lowering pH 1  for example with acid ammonium sulphate, or by adding flocculating agents e.g. alkyl sulphonic acids or poly (styrene sulphonic acid).

United States Patent ABSTRACT OF THE DISCLOSURE A method for preparingsilver halide dispersions by mixing aqueous solutions of silver nitrateand alkali metal halides in the presence of a polyoxyalkylenatedderivative of gelatin to form a silver halide-polyoxyalkylenated gelatindispersion, followed by coagulation and recovery of the dispersion.

This invention relates, in general, to photography and in particular tothe preparation of silver halide dispersions in a water-permeablematerial having protective colloid properties.

The conventional methods heretofore employed commercially for thepreparation of silver halide dispersions in a protective colloid carriersuch as gelatin involve, as essential sequential operations, thereaction of a watersoluble silver salt such as silver nitrate, with atleast one a water-soluble halide such as potassium or sodium bromine,preferably together with potassium or sodium iodide, in an aqueoussolution of a gelatin colloid peptizing agent. The dispersion of silverhalide thus formed contains water-soluble salts as a by-product of thedouble decomposition reaction, in addition to an unreacted excess ofeither of the initial salts. In such methods wherein gelatin is employedas the peptizer, the methods have been, in the past, limited in scope bythe physical properties peculiarly characteristic of gelatin. Thesephysical properties have necessitated the use of concentrations ofsilver halide and a gelatin sometimes, which are not the most desirablefor photographic products. For instance, emulsions of highconcentrations of silver halide have proved diflicult to prepare.Limitations have also been imposed upon the method regarding theconditions of dilutions during the emulsion preparation since emulsionsof very high water content are diflicult to coat and dry. Often, thetechnician has been obliged to limit the minimum concentration ofgelatin in the washing operation in which soluble salts are removed.

It has been considered desirable, in the case of photographic emulsions,particularly those to be coated on water-impermeable supports, such asfilm base, that the soluble salts present in the emulsion be reduced inconcentration. It has been the practice previously, where gelatin hasbeen used as the protective colloid, to remove the soluble materialstherefrom by setting the gel dispersion by means of chilling andnoodling the so-set dispersion and washing the noodles in cold water.Under such conditions, a minimum gelating concentration of approximately3-4% is required under practical conditions, and this limitation hashampered the operations of the emulsion maker in his preparation ofemulsions by this method. Moreover, this method has the distinctdisadvantage in that the solidification, noodle formation and washingoperations are time-consuming. Furthermore, such operations must beaccomplished in the presence of an amount of binding agent which cannotbe reduced to the extent desired. In some cases, for instance in thepreparation of photographic emulsions to be used in X-ray photography,

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a relatively large amount of light-sensitive silver salt should bepresent per unit of surface coating. In an attempt to remedy theforegoing situation, emulsion concentration has been effected by drivingoff the water present, as, for example, by evaporation. However,according to this method, the proportion of binding agent tolight-sensitive salt remains unchanged. Consequently, a moreconcentrated emulsion is only obtained at the expense of a highviscosity, which can hinder the coating of the emulsion to the extent ofnecessitating the use of special coating methods.

In an effort to overcome or otherwise mitigate the shortcomings inherentin the procedures described above, more recent methods are based onsilver halide precipitation techniques for the purpose of renderingnoodle formation as well as extended washing operations unnecessary. Insuch method, the emulsions are coagulated by adding a precipitant toremove the excess salts, with the coagulate being separated from thesupernatant aqueous solution containing the salts in dissolved form,washed with water and, after adding more gelatin solution, often alsore-dispersed by adjusting the pH to a predetermined value. It ispossible in this way to obviate the rinsing operation thus effecting aconsiderable saving in time. In addition, the emulsion character can bevaried greatly and improvements in the photographic properties can beobtained. The increasing importance of this flocculation process isillustrated by the large selection of variants of the process which havebecome known in recent times. However, the flocculation of precipitationmethods thus far proposed have been found to suffer from one or moredisadvantages which greatly reduce their over-all effectiveness. Forexample, the methods currently enjoying relatively widespread userequire relatively large quantities of precipitating agent in order toachieve silver halide-gelatin dispersions of the requisite photographicqualities. The latter consideration assumes considerable importance whenit is realized that many of the extensively used precipitating agentsare relatively expensive and in many cases inflammable therebypresenting a variety of problems with respect to storage, handling andthe like.

In addition, such methods present considerable difficulty in connectionwith the digestion and washing operations. The latter operation usuallyrequires copious amounts of water While the former operation hascharacteristically resulted in the introduction of undesirable physicalproperties into the emulsion batch. As a consequence, this situation hasnecessitated the application of certain remedial measures such ascorrective changes in the emulsion formulation. This, in turn, mayoftentimes be at the expense of properties considered desirable if notessential in the final product. Moreover, the physical ripening ofgelatin hydrolysates is not entirely satisfactory for the further reasonthat such hydrolysates may possess insuflicient carrying power andundesired sedimentation of the silver halide may thus occur. This,combined with the large amounts of water required in the washingoperation make diflicult the obtention of silver halide emulsions withthe desired silver halide content.

In accordance with the present invention, there is provided a method forpreparing silver halide dispersions which greatly reduces the amount ofprecipitating agent required for forming the silver halide dispersionand which allows the production of photographic emulsions therewith ofsignificantly increased silver content. Accordingly, it is an object ofthe present invention to provide an improved process for the preparationof silver halide dispersions in a protective colloid binder as well asthe novel products produced thereby which will not be subject to one ormore of the above disadvantages.

It is a further object of the present invention to provide an improvedprocess for the preparation of dispersions of silver halide useful inthe production of photographic, gelatino-silver halide emulsionscharacterized by outstanding improvement in silver halide colloidratios, and wherein the amount of precipitating agent required informing the silver halide dispersion is greatly reduced.

Other objects and advantages will appear'as the description proceeds.

The attainment of the foregoing objects is made possible by the presentinvention which comprises a method of preparing a dispersion of a silverhalide involving the steps of mixing together a water-soluble silversalt and at least one water-soluble halide salt in the presence of agelatin derivative which is the reaction product of gelatin with analkylene oxide in varying amounts to form thereby a dispersion of silverhalide and polyoxyalkylenated gelatin, and coagulating and recoveringthe dispersion thus formed. It is highly important to the results of thepresent invention that the coagulation, i.e., ripening andprecipitating, of the silver halide grains be effected in the presenceof the gelatin-alkylene oxide adduct, this procedure forming the salientfeature of the invention both described and claimed herein.

The term gelatin as utilized herein is intended to signify theart-accepted designation of the photographic colloid binder derived fromcollagen and comprising a proteinaceous substance. It is also intendedto include any other product substantially identical therewith, as forexample, where such product is produced synthetically. While the exactstructure of the gelatin is not known, it occurs in bones, hides, skinsand sinews and can be readily obtained from the parent substance,collagen, according to well known prior art treatments which usuallyinvolve some form of treatment such as degreasing in the case of bonestock and liming or acidnlation in the case of hide or sinew stock, toprepare the raw material for the cooking step. The above treated rawmaterial is hydrolyzed into gelatin by heating in water to yield therebya solution which is concentrated by vacuum evaporation and subsequentlyspray or drum dried to produce a gelatin product recoverable in the formof sheets, flakes, powder, pearls or woodles depending upon the mode ofprocessing.

Any form of gelatin appears to be operable for the formation of thewater soluble oxide adducts thereof which are employed in the novelsilver halide dispersions of the present invention.

The term polyoxyalkylenated gelatin is intended to include within itsscope those oxide adducts obtained by reacting gelatin with an alkyleneoxide, the latter being employed preferably in amounts ranging from 0.8to 5 parts per part of gelatin. These oxide adducts may easily andconveniently be prepared by simply heating the gelatin with theparticular alkylene oxide at a temperature ranging from 50 C. to 200 C.in the presence of a suitable catalyst. As examples of catalystmaterials found to be suitable for the preparation of the abovedescribed gelatin-alkylene oxide adducts, there may be mentioned thealkoxides, oxides and hydroxides of lithium, potassium, sodium, calcium,zinc and lead. However, the alkali metal hydroxides such as sodium andpotassium hydroxide are found to be especially suitable. Contacting ofthe alkylene oxide and gelatin may be effected in simple fashion bymerely passing a stream of the alkylene oxide through an aqueoussolution of the gelatin at temperatures within the range above stated.

The alkylene oxides which find immediate and prac tical application inproducing the water soluble, polyoxyalkylenated gelatin derivativescontemplated for use in the present invention are preferably vicinalepoxides of the following formula:

wherein R can be hydrogen, alkyl, cycloalkyl, alkaryl, aralkyl, and thelike. As examples of alkylene oxides coming within the scope of theabove formula and found to be highly suitable to the purposes of thepresent invention, there may be mentioned ethylene oxide, propyleneoxide, butylene oxide, styrene oxide, cyclohexene oxide and the like,including mixtures of two or more of the foregoing.

The term emulsification as used herein is intended to include thosesteps customarily employed in effecting silver halide-gelatin dispersionformation including the manipulative steps directed towards controllingthe grain size, grain size distribution and grain sensitivity of thesilver halide crystals thus formed.

Accordingly, the term emulsification is to be understood ascomprehending digestion of the silver halide dispersion, the latter termhaving its well known art accepted significance and conoting the usualsteps of heating the silver halide polyoxyalkylenated gelatin dispersionunder controlled conditions of time and temperature. It is to be furtherunderstood that the ripening or digestion step may be carried out in thepresence of additional ingredients such as ripening agents, stabilizers,and the like. The precipitate or flocculation of the polyoxyalkylenatedgelatin-silver halide dispersion can be achieved with the use ofconventional and well known agents. As examples of flocculating agentscontemplated for use herein there may be mentioned long chain alkylsulfonic acids such as those wherein the alkyl group contains from 12.to 20 carbon atoms, organic polymeric materials such as polymers andcopolymers of styrene sulfonic acid and the like. In the broad sense,flocculation or precipitation as used herein is intended to include mostany procedure which results in a lowering of the pH of thepolyoxyalkylenated gelatin dispersion so long as the properties desiredin the final emulsion product are not deleteriously affected thereby.For example, good results can be obtained when pH adjustment is effectedby the use of such compounds as acid ammonium sulfate and the like.

In accordance with the present invention it is found that dispersions ofsilver halide, wherein the precipitation of the silver halide materialis effected in the presence of a gelatin alkylene oxide adduct of thetype described above, are highly suitable for use in the preparation ofphotographic, gelatino-silver halide emulsions of significantlyincreased silver contents with exceptional silver halide/ gelatin ratiosbeing obtained therewith. Moreover, with respect to the actual silverhalide precipitating step, it is found that the presence of the abovedescribed polyoxyalkylenated gelatin derivatives greatly reduces theamount of the precipitating agent which would otherwise be necessary toachieve the requisite degree of silver halide deposition. This, ofcourse, renders the procedure provided by the present invention highlydesirable from an economical standpoint for the reasons more fullyenumerated hereinbefore. In addition, the use of precipitating orflocculating agents can be dispensed with entirely according to thepresent invention since the use of the above describedgelatin-polyoxyalkylenated adducts permits silver halide precipitationto be accomplished by merely changing the pH of the dispersion, i.e.,rendering same acidic.

The dispersion thus formed, as pointed out herein'before, is availablefor effective use in photographic emulsion compositions. If desired,following precipitate formation, the dispersion may be washed preferablyemployin'g acid pH conditions and then re-dispersed in a suitableorganic protective colloid vehicle such as unmodified gelatin. Thewashing operation may be repeated as often as is considered desirablealthough it is found that washings in excess of three or four are rarelynecessary for achieving optimum results.

In carrying out the present invention, the silver halide is prepared inan aqueous solution of polyoxyalkylenated gelatin as the peptizingagent. The silver halide may be prepared by any of the normal methodsfor the preparation of silver halide dispersions such as for example byintroducing a stream of an aqueous solution of silver nitrate and astream of an aqueous solution of at least one alkali metal halidepreferably potassium bromide and potassium iodide into a constantlyagitated solution of the peptizing agent. Alternatively however, thepolyoxyalkylenated gelatin solution may "be combined with one of thereactants, i.e., the nitrate or halide, and the other reactants may beintroduced thereinto with stirring. Following silver halideprecipitation, any ripening or digesting operations considered advisablemay be effected. The dispersion of silver halide is then coagulated bylowering the pH 0 fthe solution to a value below 6 and preferably withinthe range of 2 to 4 and/or by the addition of fiocculating agents of thetype described above. The silver halide-polyoxyalkylenated gelatindispersion forms granules and quickly settles from the liquid portion ofthe mass. The coagulum thus formed may be separated from the liquid byany one of several techniques such as for example by removing thesupernatant mother liquor from the receptacle in which the mass incontained by means of a siphon or by decantation. For some types ofemulsions the coagulum thus obtained may be used directly without anyfurther treatment. However, where it is desired to further reduce thecontent of soluble salts which may be present, any one of severalmethods may be employed. For instance, the coagulum may be rinsed bytreatment with cool water preferably with the pH thereof adjusted toapproximate the range of the isoelectric point of the polyoxyalkylenatedgelatin. As an alternative method, the coagulum may be redispersed inwater made slightly alkaline using elevated temperatures; the solutionis then stirred until redispersion of the coagulum is effected,following which the silver halide-polyoxyalkylenated gelatin dispersionmay again be precipitated by addition of sufiicient acid to reduce thepH to the coagulation point of the gelatin derivative. The foregoingprocedure of redispersion and coagulation may be repeated as many timesas is deemed necessary. Alternatively, the washing may be effected byreducing the pH to values below approximately 2 whereupon re-coagulationmay be effected by the addition of a suitable alkaline material such assodium or ammonium hydroxide.

The coagulum from Which the water soluble salts have been removed,either by decantation or one of the methods described above, may then beredispersed in a suitable organic protective colloid such as gelatin,either modified or unmodified, to produce a composition uitable for useas a photographic emulsion. The pH values necessary for coagulation andredispersion depend in part on the dissocation constants of the ionicmaterials present in the silver halide dispersion and can, of course,vary over a wide range. In some cases, sedimentation will occur at anypH value although redispersion can be effected, for example, by theaddition of a water-permeable organic colloid binder. Centrifugation, ifnecessary, can also be applied.

The emulsions of this invention may be coated onto various types ofrigid or flexible supports, for example, glass, paper, metal, polymericfilms of both the synthetic types and those derived from naturallyoccurring products, etc. Especially suitable materials include paper;aluminum; polymeth-acrylic acid methyl and ethyl esters; vinyl chloridepolymers; polyvinyl acetals; poly-amides such as nylon; polyesters suchas the polymeric films derived from ethylene glycolterephthalic acid;and cellulose derivatives such as cellulose acetate, triacetate,nitrate, propionate, butyrate, acetate-propionate, or acetatebutyrate.

The ligh-sensitive material of the photographic emulsions comprises acompound of silver, for example, one or more of the silver halides ofwhich silver chloride, silver bromide and silver iodide are examples.The preferred silver halide emulsion comprises a silver iodobromideemulsion. It will be understood that preferred silver halide emulsionsof varying halide concentration may be advantageously employed.

The invention will be illustrated in greater detail in conjunction withthe following specific examples which set out representativepreparations of the novel emulsions of this invention, which, however,is not limited to the details therein set forth and is intended to beillustrative only.

EXAMPLE I 900 grams of silver nitrate are dissolved in 5400 cc. of anaqueous ammoniacal solution and added over a period of '35 minutes to anaqueous solution of '22 grams of potassium iodide, 680 grams of ammoniumbromide and 40 grams of a propylene oxide-gelatin reaction product.Approximately 35 milliliter of a 15 percent solution ofpoly-(sulfostyrene) is added and after a 5 minute digestion period, thepH is lowered by acid addition to efiFect coagulation of the dispersion.The coagulum thus obtained is washed four times at a low pH and 500grams of an unmodified gelatin are added in solution with stirring. Whenthe precipitation is dispersed, the solution is brought up to volume bygelatin addition. Preservatives are added, and the mass is digested at61 C. with agitation while the necessary ripeners and stabilizers areadded. The emulsion is then processed and coated as usual. Theimprovements provided by the present invention are clearly manifested bythe fact that only onethird of the amount of poly (sulfostyrene)required in conventional emulsion preparation methods is necessary toachieve precipitation of the silver halide dispersion. Moreover,photographic elements prepared with samples of the emulsion of the aboveexample are characterized by highly improved silver contents whencompared to conventional photographic emulsion compositions.

EXAMPLE H 600 grams of silver nitrate in 470 cc. of distilled Water areadded over a period of 20 minutes to 1250 cc. of a previously preparedsolution of 440 grams of potassium bromide, 24 grams of potassium iodideand 67 grams of a butylene oxide-gelatin reaction product. After a 3minute digestion at 67 C. the pH is lowered by acid addition and cc. ofan 11 percent solution of poly (sulfostyrene) is added as aprecipitating agent. After coagulation takes place, the precipitate iswashed four times and redispersed in gelatin. This solution is broughtup to volume by gelatin addition and digested while the necessaryripeners, stabilizers and preservatives are added. Results similar tothose of Example I are obtained as regards the amount of precipitatingagent required, and the silver halide-gelatin ratio characterizing theemulsion product.

EXAMPLE III An emulsion is produced in the manner described in ExampleI, with the exception that an ethylene oxidegelatin reaction product isused. Again, precipitation is effected with a minimal amount ofprecipitating agent. Results similar to those of the preceding examplesare obtained.

EXAMPLE IV An emulsion is made in the manner described in Example I,with the exception that acid ammonium sulfate is added after thedigestion period as a precipitating agent. Results similar to those ofthe preceding examples are obtained.

Like results are obtained When the procedure of the foregoing examplesis repeated using as polyoxyalkylenated gelatin derivatives thematerials obtained by reacting gelatin with ethylene oxide, styreneoxide and cyclohexene oxide, i.e., precipitation was effected utilizingsignificantly reduced quantities of precipitating agent, while thesilver halide dispersions produced therewith displayed exceptionalphotographic properties in addition to significantly improved silvercontents.

One of the salient advantages of the present invention is that theprocess of precipitating the silver halides in the presence of thepolyoxyalkylenated gelatin eliminates the necessity of chilling andnoodling the emulsion prior to the removal of the soluble salts bywashing. This conventional method of chilling, noodling and washingnecessitates the use of large amounts of water. There are at least threeprimary objections to such a procedure. One is that the water used forwashing has to be quite pure and free of contamination and thisnecessitates elaborate and costly filtering operations. The secondobjection is that the noodles take up a considerable amount of the washwater and require higher concentrations of gelatin than would normallybe necessary if this water was not absorbed. Thus, when conventionalwashing methods are employed emulsions have to be made with at least a 4percent gelatin content whereas the instant process requiresconsiderably less gelatin, e.g., of the order of 1.5 to 2.5 percent. Inlarge manufacturing operations this involves considerable savings. Thethird objection is that the high water content of the conventionallywashed emulsions militates against using higher coating speeds which inturn necessitates faster drying means. A further disadvantage of theconventional emulsion making method is that the silver nitrate solutionscan only be used in limited concentration, that is the volume ofsolution must be kept small as determined by the gelatin content whereasin the instant process higher volumes and less concentrated solutionscan be used since all the liquid is removed in the precipitation. Thislarger choice of solution volume may in turn allow greater control ofphotographic characteristics, i.e., grain size, etc.

A further advantage of the invention is that it allows one to makegelatin emulsions with low gelatin content since substantially all ofthe water used in making the emulsion is removed from the precipitatedemulsion. Since practically no solvent is required for redispersion, thegelatin concentration for coating can be easily controlled by theaddition of water. When noodled emulsions are washed by the conventionalprocedure the minimum concentration required for proper setting oft theemulsion is approximately 4 percent. In order to maintain thispercentage as well as the silver halide/gelatin ratio required for thefinished film, the formulae and procedures are sometimes hampered byserious operational limitations. During the conventional washingoperation the water content of the emulsion is increased whilesubstantially all of 0 the water can be removed by precipitation. Thisofiers the simple means of increasing the solids content of the emulsionafter they have been made. Thus, one can obtain emulsions that have ahigher silver halide/gelatin ratio than those obtained with emulsionswhich have been washed by the conventional method.

The emulsions of the present invention may be chemically sensitized byany of the accepted procedures. For example, the emulsions may bedigested with naturally active gelatin or sulphur compounds can beadded. Chemical sensitization may also be effected by the use of noblemetal salts, reducing agents and the like. Optical sensitization may beeffected by the use of cyanine and merocyanine dyes, etc.

Other conventional additives such as speed-increasing agents,anti-foggants, restrainers, accelerators, preservatives, coating aidsand/ or stabilizers may also be included in the emulsion compositions ofthe present invention.

Various modifications of the present invention as set forth in thepreceding examples will become apparent to those skilled in the art suchas the use of equivalent materials not necessarily disclosed in thespecification. It is not intended, therefore, that the invention belimited except as necessitated by the appended claims.

What is claimed is:

1. A method of preparing a silver halide dispersion comprising the stepsof mixing together a Water soluble silver salt and at least one watersoluble halide salt in the presence of a polyoxyalkylenated derivativeof gelatin to form thereby a dispersion of silver halide and saidpolyoxyalkylenated gelatin derivative, coagulating and recovering thedispersion thus formed.

2. A method according to claim 1 wherein the polyoxyalkylenatedgelatin-silver halide coagulum thus recovered is redispersed in aprotective colloid therefor.

3. A method according to claim 2 wherein the protective colloid isgelatin.

4. A method according to claim 2 wherein the polyoxyalkylenatedgelatin-silver halide coagulum thus recovered is washed prior toredispersing same in the said protective colloid therefor.

5. A method according to claim 1 wherein the polyoxyalkylenated gelatinderivative is the reaction product of gelatin with a compound selectedfrom the group consisting of ethylene oxide, propylene oxide, butyleneoxide, styrene oxide, cyclohexene oxide and mixtures thereof.

6. A method according to claim 1 wherein coagulation is effected byreducing the pH of the silver halide dispersion to a value below 6.

7. A method according to claim 1 wherein coagulation is etfected by theaddition of poly(sulfostyrene) to the silver halide dispersion.

3. A method according to claim 1 wherein coagulation is effected by theaddition of acid ammonium sulfate to the silver halide dispersion.

References Cited UNITED STATES PATENTS 2,131,145 9/1938 Schlack 8543,153,593 10/1964 De Pauw 9694 3,168,403 2/1965 Himlmelrnann et al 9694FOREIGN PATENTS 811,153 4/1959 Great Britain.

J. TRAVIS BROWN, Primary Examiner.

